It has been a very long time since I have been on a cruise, but I was able to connect with a diver and do do some diving in Ketchikan, Alaska from our cruise ship. Some of my most memorable dives of all time for the size and quantity of life that I saw! And the water was warmer than my usual spots in Lake Huron!
When I saw that the cruise offered a snorkelling excursion, I went online and tracked down a local diver who picked me up from the cruise ship, set me up with tanks and weights, and took me diving for the day. I did this planning from home, a few weeks before departure.
While setting up, breaking down and during our surface interval, we were within sight of the snorkelling excursion, so were able to keep an eye on them for their departure back to the ship. I ended up back in plenty of time, and even got to do some of my own sightseeing around town.
Logistically, bringing dive gear on a cruise can be a bit challenging-- especially big, bulky cold water gear. As the gear was drying, there was not much space left in the cabin for us.